The present disclosure relates to the correction of the frequency response and especially of the phase frequency distortions in the vector/signal analyzers or in the similar devices, where the input and output signals are in different frequency ranges.
Wireless networking systems have become a prevalent means in the communication industry. In such systems it is very important to measure/analyze with a high degree of accuracy the various properties of a transmitted/received modulation signal. Therefore a large and perpetually increasing demand exists for high precision RF vector/signal analyzers.
A typical block diagram of a vector/signal analyzer is shown in FIG. 1. The input signal is conditioned by down converter 100 and analog to digital converter (ADC) 101. The down converter 100 transfers the part of the input signal spectrum to be analyzed to the operational frequency range of the ADC 101. The ADC 101 transforms incoming continuous signal into a sequence of digital samples. After this conditioning, a processor 102 carries out the necessary analysis of the properties of the processed signal with the presentation of the received results at the display 103.
It is important for the down converter 100 not to create spurious responses, which may substantially distort the processed signal. To attain such a purpose, a conventional down converter contains usually several conversion stages (three or four) with an appropriate selection of the local oscillators frequencies. At each conversion stage a filter is used to separate out the desired frequency components. These filters inevitably introduce frequency distortions in the processed signal. To achieve a high degree of measurement accuracy in a vector/signal analyzer it is necessary to compensate the frequency distortions that emerge in the down converter 100.
The known methods of frequency responses measurement are based on a comparison of the output signal of the device under test with the input signal or with a duplicate of the input signal. Such an approach can be used to find the amplitude frequency distortions in a vector/signal analyzer. However, since the input and the output frequency ranges of the down converter 100 are different, it is impossible to compare the phases of sine wave components in the input signal and output signal: the difference between the phases varies in time continuously.